Cellular communications systems are commonly employed to provide voice and data communications to a plurality of mobile units or subscribers. Analog cellular systems, such as designated AMPS, ETACS, NMT-450, and NMT-900, have been deployed successfully throughout the world. More recently, digital cellular systems such as designated IS-54B in North America and the pan-European GSM system have been introduced. These systems, and others, are described, for example, in the book titled Cellular Radio Systems by Balston, et al., published by Artech House, Norwood, Mass., 1993.
As illustrated in FIG. 1, a cellular communication system 20 as in the prior art includes one or more mobile cellular radiotelephones 21, one or more base stations 23 and a mobile telephone switching office (MTSO) 25. Although only three cells 36 are shown in FIG. 1, a typical cellular network may comprise hundreds of base stations, thousands of cellular radiotelephones and more than one MTS0. Each cell will have allocated to it one or more dedicated control channels and one or more voice channels. A typical cell may have, for example, one control channel, and 21 voice/data, or traffic, channels. The control channel is a dedicated channel used for transmitting cell identification and paging information. The traffic channels carry the voice and data information.
The MTSO 25 is the central coordinating element of the overall cellular network 20. It typically includes a cellular processor 28, a cellular switch 29 and also provides the interface to the public switched telephone network (PSTN) 30. Through the cellular network 20, a duplex radio communication link 32 may be effected between two cellular radiotelephones 21 or, between a cellular radiotelephone 21 and a landline telephone user 33. The function of the base station 23 is commonly to handle the radio communication with the cellular radiotelephone 21. In this capacity, the base station 23 functions chiefly as a relay station for data and voice signals. The base station 23 also supervises the quality of the link 32 and monitors the received signal strength from the cellular radiotelephone 21.
One visible feature of a typical base station 23 is the antenna tower 35. In order to achieve a reasonable coverage area, the antennas 63, 65 are typically mounted at some distance above the ground. Referring now additionally to the prior art schematic plan view illustration of FIG. 2A, in rural areas the towers 35 are commonly located at the center of a cell 36 thereby providing omni-directional coverage. In an omni-directional cell, the control channel(s) and the active voice channel(s) are broadcast in all areas of the cell--usually from a single antenna. Where base stations 23 are more densely located, a sectorized antenna system may be employed as in the prior art, and shown by the schematic diagram of FIG. 2B. Sectorization requires directional antennas 70 having, for example, a 120 degree radiation pattern as illustrated in FIG. 2B. Each sector 71 is itself a cell.
Conventional analog cellular systems generally employ a system referred to as frequency division multiple access (FDMA) to create communications channels. As will be understood by those skilled in the art, cellular radiotelephone systems generally communicate using radiotelephone communications signals communicated over a spectrum of carrier frequencies. As a practical matter, radiotelephone communications signals, being modulated waveforms, typically are communicated over predetermined frequency bands in the spectrum of carrier frequencies. In an FDMA system, "channels" are discrete predetermined frequency bands over which cellular radiotelephones communicate with a cellular base station. In the United States, for example, Federal authorities have allocated to cellular communications a block of the UHF frequency spectrum further subdivided into pairs of narrow frequency bands, a system designated EIA-553 or IS-19B. Channel pairing results from the frequency duplex arrangement wherein the transmit and receive frequencies in each pair are offset by 45 Mhz. At present there are 832, 30-Khz wide, radio channels allocated to cellular mobile communications in the United States.
The limited number of available frequency bands presents several challenges as the number of subscribers increases. Increasing the number of subscribers in a cellular radiotelephone system requires more efficient utilization of the limited available frequency spectrum in order to provide more total channels while maintaining communications quality. This challenge is heightened because subscribers may not be uniformly distributed among cells in the system. More channels may be needed in particular cells to handle potentially higher local subscriber densities at any given time. For example, a cell in an urban area might conceivably contain hundreds or thousands of subscribers at any one time, easily exhausting the number of frequencies available in the cell.
For these reasons, conventional cellular systems employ frequency reuse to increase potential channel capacity in each cell and increase spectral efficiency. Frequency reuse involves allocating frequencies to each cell. Cells employing the same frequencies are geographically separated to allow mobile units in different cells to simultaneously use the same frequency without interfering with each other. By so doing, many thousands of subscribers may be served by a system of only several hundred frequency bands.
Time division multiple access (TDMA) systems provide potentially greater channel capacity and spectral efficiency through more efficient allocation of system time resources. A TDMA system may be implemented by subdividing the frequency bands employed in conventional FDMA systems into sequential time slots, so that a TDMA channel comprises a particular frequency and time slot combination. An example of a system employing TDMA is the dual analog/digital IS-54B standard employed in the United States, in which each of the original frequency bands of EIA-553 is subdivided into 3 time slots.
Code division multiple access (CDMA) systems have been proposed as an alternative method of increasing channel capacity and spectral efficiency. CDMA systems employ spread-spectrum techniques previously developed for military communications. Subscribers are assigned one or more unique spreading codes, each of which represent a channel in the CDMA system. In one form of CDMA, transmission involves the direct modulation of a data-modulated carrier frequency signal by a digital spreading code waveform, a technique known as direct-sequence modulation. In another form of CDMA, a data-modulated carrier frequency signal is modulated by a frequency-hopping signal which changes frequency at fixed time intervals according to a spreading code. Yet another form of CDMA represents a combination of these direct-sequence and frequency-hopping techniques. These and other forms of CDMA are described in Chapter 8 of the book entitled "Mobile Communications Engineering," by William C. Y. Lee, published by McGraw-Hill, 1992.
The transmitted signal in these CDMA systems is a coded signal which is "spread" away from the original carrier frequency across the frequency spectrum. The coded transmission is recovered, or "despread," by a receiver operating with a matching spreading code. A unique aspect of CDMA is that each subscriber operates across the available spectrum, instead of operating on discrete frequency bands. Thus, CDMA potentially provides for more efficient spectrum utilization and for gradual and uniform degradation of communications quality among all users as system becomes overloaded. CDMA is also theoretically more immune to certain types of narrow-band interference.
The performance of TDMA and CDMA systems is improved by synchronization of time slots or spreading codes among base stations. For example, synchronization eases handoff between cells, allows reduced "guard times" between time slots and reduces interference caused by overlap of TDMA time slots or CDMA spreading codes. Methods for synchronizing base stations are disclosed in U.S. Pat. No. 5,293,423 to Dahlin et al., U.S. Pat. No. 5,363,379 to Chuang et al., U.S. Pat. No. 5,410,588 to Ito, International Application No. PCT/FI93/00228 to Vikamaa (filed May 27, 1993), European Patent Application No. 93307951.9 to Matsuno (filed Jun. 10, 1993), European Patent Application No. 93305417.3 to Hashimoto (filed Sep. 9, 1993) and Japan Patent Application No. 91264432 to Kazuyuki (filed Oct. 14, 1991).
Conventional TDMA cellular systems generally allocate groups of frequencies for each channel according to a fixed frequency reuse plan. The analog AMPS system used in the United States, for example, employs a 21-cell reuse pattern to achieve a desired signal to interfering noise ratio (C/I) of 17-18 dB when the system is fully loaded. This reuse pattern guarantees a minimum separation of approximately 2 cell diameters between base stations employing the same group of carrier frequencies. The digital GSM system typically employs a seven- or nine-cell reuse pattern, maintaining comparable signal quality through supplemental digital coding techniques.
Apart from the generally undesirable alternative of increasing the number of frequency bands, other techniques for increasing potential cell channel capacity and spectral efficiency include adaptive allocation of channels between stations and the use of microcell techniques.. Adaptive channel allocation dynamically assigns channels between radiotelephones and base stations based upon loading and other system parameters. For example, Swedish Application No. 9301695-4 to Wallestedt (filed May 17, 1993), discloses the use of an adaptive channel allocation (ACA) algorithm which assigns radiotelephones to base stations according to signal quality, channel availability and other indices. Microcell techniques attack the problems of insufficient local channel capacity and spectral inefficiency by decreasing cell size, thus lowering the probable number of subscribers in a particular cell and allowing more frequency reuse in a given geographical area.
As the number of subscribers using a cellular radiotelephone system increases, there is an ever present need for greater cell channel capacity and more efficient spectrum utilization. The fixed frequency reuse plans of conventional systems such as AMPS and GSM may provide insufficient local channel capacity by assigning the same group of frequencies for all time slots for a given reuse group. These fixed frequency groupings also often result in poor spectrum utilization as demand varies across the cellular radiotelephone system. Although adaptive channel allocation may more efficiently assign existing channels, it generally does not increase the potential number of channels available in a given cell. Microcell techniques may decrease channel demand in each cell, but may not be cost effective for large-scale cellular radiotelephone systems because more base stations may be required.